Abstract: This is the first comprehensive book that explores the subject of federalism from the perspective of comparative constitutional studies and at the same time places a strong emphasis on how federal systems work in practice. This focus is reflected in the book's two most innovative elements: First, it analyses, from a comparative point of view, how government levels exercise their powers and interact in several highly topical policy areas like social welfare, environmental protection or migrant integration. Second, the book incorporates text case law boxes discussing seminal judgments from federal systems worldwide and thus demonstrates the practical impact of constitutional jurisprudence on policy-makers and citizens alike.